Thermoelectric generator devices convert heat energy from a heat source directly to electrical energy. In one type of thermoelectric generator, the electrical energy is generated by electrochemically expanding alkali metal across a solid electrolyte. Such generators, wherein sodium metal is employed as the working substance, have been described in U.S. Pat. Nos. 3,458,356 and 4,510,210, and are commonly referred to as "sodium heat engines" (referred to herein as "SHE"). This type of thermoelectric generator is discussed herein as exemplary of one type of generator in which the article of this invention may be suitably used.
The sodium heat engine generally comprises a closed container separated into a first and second reaction zone by a solid electrolyte. Liquid sodium metal is present in the first reaction zone (i.e., on one side of the solid electrolyte). In the second reaction zone (i.e., on the other side of the solid electrolyte), a permeable, electrically conducting electrode is in contact with the solid electrolyte. During operation of such a device, a heat source raises the temperature of liquid sodium metal within the first reaction zone to a high temperature and corresponding high vapor pressure, which creates a sodium vapor pressure difference across the solid electrolyte. In response to this pressure difference, the elemental sodium gives up electrons to the negative electrode in contact with the sodium metal and the resulting sodium ions migrate through the solid electrolyte. The electrons, having passed through an external load, neutralize sodium cations at the permeable, positive electrode-solid electrolyte interface. Elemental sodium metal evaporates from the permeable electrode and migrates through the low pressure second reaction zone to a low temperature condenser. The condensed liquid sodium may then be returned back to the higher temperature first reaction zone.
In the thermoelectric generator system just described, the electrode on the surface of the electrolyte from which the alkali metal ions emerge is a positive electrode and must be present in order to transfer electronic charge from the external circuit to the alkali metal ions. This completes the electrochemical circuit required for operation of the generator. The operation of such thermoelectric generator systems require electrodes possessing special properties, some of which are difficult to optimize simultaneously. For example, it is necessary for efficient generator operation that the positive electrode conduct electrons from the electrical load to a broad surface of the electrolyte, doing so with low electrical resistance. At the same time, it is also necessary for the positive electrode to permit the passage of alkali metal atoms from the electrolyte-electrode interface through the electrode to the opposite electrode surface, from which they may pass to the condenser. While the former requirement is more likely to be attained by dense, thick electrodes to promote low resistance, the latter requirement suggests thin, permeable electrodes to promote the easy passage of the alkali metal through the electrode. Additionally, the electrodes must be relatively unreactive with the alkali metal and have low vapor pressure to prevent their loss through evaporation in the high temperature, high vacuum environment in which they operate. Still further, the electrode material must have a thermal expansion coefficient offering a fair match to that of the electrolyte substance. This is necessary in order to prevent delamination of the electrode from the electrolyte which could result from differential expansion and contraction of the electrode and electrolyte materials during the heating and cooling cycles to which such systems are exposed during use.
U.S. Pat. No. 4,049,877, to Saillant et al, is directed to a thermoelectric generator wherein the improvement comprises employing, as the electrode, a porous metal film deposited on the solid electrolyte by chemical deposition specifically chemical vapor deposition. Among the metals taught as suitable for use as the electrode are molybdenum, tungsten, chromium, nickel and iron. Cole, in U.S. Pat. No. 4,175,164, teaches that the surface configuration of metal electrodes formed, e.g., by chemical deposition techniques (such as those in the above Saillant et al patent) may be modified by subsequently exposing such deposited electrodes to oxidizing conditions, followed by reducing conditions. It is suggested by Cole that these conditions effect an oxidation, reduction and consequent redeposition in the already deposited electrode material, e.g., molybdenum, and modify the surface configuration which makes it desirably more porous, thus providing improved electrode efficiency.
It may be that sodium molybdates may be desirably formed on the surface and in the pores of Cole's electrode in the presence of sodium and oxygen (from the oxidizing conditions described in that patent), resulting in the excellent initial power of the Cole electrode. However, during operation of sodium heat engine it is believed that any molybdates formed at the molybdenum surface and in the pores of the Cole electrode evaporate rapidly, leaving a solid, less permeable molybdenum electrode. It is believed that the loss of these liquid phases through evaporation or decomposition leads to a dramatic decrease in power output in a short period of time, e.g., 50-150 hours as shown in FIG. 1.
U.S. application Ser. No. 166,133 disclosed above is directed to an article suitable for use in thermoelectric generators. The article comprises a thin film electrode comprising molybdenum oxide on solid electrolyte. The molybdenum oxide is deposited by physical deposition of molybdenum in an atmosphere comprising at least 10% oxygen by volume. This electrode, as compared to that of Cole, is disclosed to incorporate oxygen substantially uniformly throughout the bulk of the electrode, probably as some form of molybdenum oxide. Thus it has been described as being able to maintain its power over a relatively long period of operation, particularly in comparison to the Cole electrode, as also shown in FIG. 1.